The large and small Magellanic clouds, which are companion galaxies to the Milky Way, glow above the antennas of ALMA, a radio telescope in Chile. Each galaxy contains a few billion stars, and lie within about 200,000 light-years of the Milky Way. The Magellanic clouds are visible the unaided eye, but only from the southern hemisphere and the lowest latitudes of the northern hemisphere. [C.Malin/ESO]

About two dozen satellite galaxies orbit the Milky Way. By far the brightest are the Large and Small Magellanic Clouds, both of which are within about 200,000 light-years of Earth. They’re best seen from the southern hemisphere, where they look like small, glowing clouds.

In the past decade, our view of these two galactic neighbors has undergone a pretty big makeover.

Astronomers used to think the Magellanic Clouds orbited the Milky Way once every one or two billion years. But in 2006, observations by Hubble Space Telescope suggested that the clouds had a wider orbit and thus a longer orbital period. And last year, new Hubble observations found that this period is at least four billion years and possibly much longer.

Meanwhile, other studies show that only a few percent of giant galaxies like the Milky Way boast a pair of bright, star-making satellites as close-in as the Magellanic Clouds. The Milky Way’s other satellites are dead — they’re devoid of any star-making gas, so they no longer give birth to new stars.

Put it all together and a new view emerges. The Magellanic Clouds are bright and vigorous, full of gas and new stars, only because until recently they’ve avoided the Milky Way, which steals gas from its satellites. And the two clouds probably orbit each other. Each galaxy’s gravity stimulates the birth of bright new stars in the other, making both of them stunning celestial sights.

More tomorrow.

Script by Ken Croswell, Copyright 2014

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